The present invention relates to a gas generator and a method for influencing a gas flow in a gas generator.
Such a gas generator is for instance suitable for an application in an occupant protection system like an airbag system in a motor vehicle for providing gas that is used for inflating an airbag. Such gas generators have to satisfy safety requirements to which amongst other things also passing the so-called bonfire test (a fire test) belongs. Thereby, the gas generator has to be designed and produced such that it is not fragmented by the heat of a fire, i.e., does not break apart. These safety requirements are prescribed by the motor vehicle producer as well as by the legislature for the admission of gas generators in motor vehicles by the Federal Institute of materials research and test.
Three or four possible positions of a gas generator are provided with respect to a flame for the bonfire test. For example, in case of a tubular gas generator four positions are to be tested:
1. a position in which the igniter of the gas generator is oriented towards the flame,
2. a position in which the igniter of the gas generator is oriented away from the flame,
3. a position in which the generator axis lies horizontally and a flame impingement of the gas generator takes place in the middle of the gas generator, and
4. a position in which the generator axis is in turn arranged horizontally, the flame impingement however occurs at a location of the gas generator being apart from the igniter.
It is known from the state of the art to employ an early ignition means (a so-called auto ignition material) in the form of pills or granules for solving the so-called bonfire problem (i.e. of the flame impingement of the gas generator while avoiding fragmentation of the same at the same time). The early ignition means comprises a lower self ignition temperature (auto ignition temperature) than a gas kit that is used in the gas generator for the actual gas production. For instance, self ignition means having a self ignition temperature of 160° C. are known. Usually, employed pyrotechnical gas kits are however much more heat resistant and comprise a higher ignition temperature or a higher exothermic reaction temperature than an early ignition means. For instance, gas kits having an exothermic reaction temperature of 260° C. are known.
One function of an early ignition means consists in causing a controlled conversion of the actual gas kit by means of an early ignition. The notion “controlled conversion” thereby means a controlled and reasonable burning rate of the gas kit. The burning rate of the gas kit increases with temperature of the gas kit before conversion. The higher the burning rate of the gas kit, the higher the internal pressure inside the gas generator. In case the internal pressure in the gas generator or in the burning chamber exceeds the usual measure, the danger of a fragmentation of the gas generator increases. This is particularly the case when the structure material of the gas generator is a weakened one due to a strong heating anyway. Thus, it is known from the state of the art that by means of a strong heating of the structure material of the gas generator in a bonfire test the structural performance of the structure material is lowered. By means of an early ignition of the early ignition means and an early conversion of the actual gas kit caused herewith at relatively low starting temperature, a load acting onto the structure material of the gas generator is kept low as a consequence of the lower internal gas pressure in the gas generator, so that the danger of a fragmentation is decreased.
The above stated function can be fulfilled particularly well by the early ignition means in case the heat acting onto the gas generator upon flame impingement of the gas generator during the bonfire test can rapidly get to the early ignition means by means of heat transfer. This is particularly the case when an advantageous test position exists, for instance in case the igniter is oriented towards the flame. Due to the distance of the early ignition means to the flame or the fire, respectively, being small in this position, the self ignition temperature of the self ignition means can be rapidly reached by advantageous heat transfer.
In case of a disadvantageous position during the bonfire test like for instance a horizontal alignment of the gas generator and a flame impingement at a location of the gas generator facing away from the early ignition means or the igniter, respectively, a delayed activation of the early ignition means may result, since a relatively large distance between the flame and the early ignition means causes a long period of time for a transfer of heat to the early ignition means. This means, that in this case the gas kit is already relatively hot upon ignition so that a higher burning rate and therefore a higher internal pressure in the burning chamber of the gas generator is reached. In addition, the generator housing has significantly lost rigidity due to the increased temperature. In the worst case a fragmentation of the gas generator may occur during this part of the bonfire test.
From U.S. Pat. No. 5,738,372 a gas generator is known that comprises a supply connection, through which an interior space of the gas generator can be filled with gas being under pressure in the interior space. After filling the interior space, the supply connection can be closed by a sealing that is deformed in case of high temperatures or melts. Hereby, the supply connection can be set free again so that it represents a safety valve of the gas generator that is opened in case of high temperatures. The gas stored in the gas generator does then however not leave the gas generator at the gas outlet but at the supply connection of the gas generator.
It would be advantageous to provide for a gas generator that passes the bonfire test in an especially advantageous manner.